Man pages matching name curlopt_ssl_enable_alpn in section 3:

CURLOPT_SSL_ENABLE_ALPN.3.en	debian 10 / libcurl4-doc
CURLOPT_SSL_ENABLE_ALPN.3.en	debian 11 / libcurl4-doc
CURLOPT_SSL_ENABLE_ALPN.3.en	debian 12 / libcurl4-doc